1. Field of the Invention
The present invention relates to a liquid crystal display (hereinafter abbreviated LCD) which realizes a high brightness and improves viewing angles.
2. Discussion of the Related Art
Lately, LCDs have been widely used as planarized plate displays. As techniques for fabricating thin film transistors using amorphous silicon are developed, the image quality of these LCDs is greatly improved. Thus, LCDs are widely used in notebook computers, personal computer monitors, and the like.
FIG. 1 shows a cross-sectional view of an LCD according to a related art. Referring to FIG. 1, the LCD includes a back light unit al for generating and uniformly supplying light, and a lower polarizing plate 110 for transforming a polarization characteristic of the supplied light from the back light unit al to a given direction. The LCD also includes a lower substrate 120 on which thin film transistors (not shown) are formed to control the transmissive rate of the light polarized by the lower polarizing plate 110. A liquid crystal layer 130 changes a polarization direction of the light whose transmissive rate has been controlled by the lower substrate 120. An upper substrate 140, on which a color filter layer 142 is disposed, causes the polarized light to have a specific color wavelength matching a corresponding color filter. An upper polarizing plate 150 changes the polarization characteristic of the light having passed through the upper substrate 140.
The back light unit a1 is constructed with a light source 102 for generating light, and a light-guiding plate 104 for guiding the light generated from the light source 102 to a liquid crystal panel uniformly. A reflection plate 100 under the light-guiding panel 104 reflects incident light from the bottom or side of the light-guiding plate 104 toward the liquid crystal panel.
In the above-constructed related art LCD, the light generated from the light source 102 passes through the light-guiding plate 104, and is uniformly polarized after passing through the lower polarizing plate 110.
A polarizing direction of the light having been polarized through the lower polarizing plate 110 is changed at the thin film transistor (not shown) on the lower substrate 120 and the liquid crystal layer 130. Namely, the polarization direction of the polarized light is selectively revolved according to a state of the transistor when passing through the liquid crystal layer 130. Then, the changed light passes through the color filter layer 142 of the upper substrate 140, which only transmits wavelengths of the specific colors corresponding to the respective color filters.
An image is realized when the light having a specific color transmitted by the color filter layer 142 proceeds to the upper polarizing plate 150. The upper polarizing plate 150 only transmits the light whose polarized direction has been revolved by the liquid crystal. In this case, the polarizing characteristic of the upper polarizing plate 150 is perpendicular to that of the lower polarizing plate 110.
Thus, the basic principle of the LCD is that the polarized direction of the light is revolved after the polarized light from the lower polarizing plate 110 passes through the liquid crystal layer 130. The length that the light travels in the liquid crystal is important. When the liquid crystal is placed between the upper and lower polarizing plates 150 and 110, the thickness of the liquid crystal changes the polarized direction of the light according to the following formula.(Δn)Z=λ/4   (I)wherein Δn is a difference of a refractive index between a perpendicular direction and a parallel direction of the proceeding direction of the light in double refraction, Z is a thickness of the liquid crystal, and λ is a wavelength of the light.
In this case, once the proceeding direction of the light is perpendicular to the liquid crystal layer, the proceeding length of the light becomes z to show the same result of the formula (I). Thus, the cut-off of the light by the vertical polarizing plate becomes very effective.
However, when the light proceeds at a slant (i.e., an acute angle to perpendicular), the proceeding (or path) length of the light becomes longer. Thus, the upper polarizing plate is unable to cut off the light precisely due to the change of the polarized direction of the light. Hence, the optimal way to achieve cut-off of polarized light is to improve the straightness (with respect to perpendicular) of the light passing though the liquid crystal.
Unfortunately, the LCD according to the related art limits the viewing angles of the LCD once the straightness of the light is elevated.